Abstract
Abstract. The water chemistry of streams and precipitation in the province of Jämtland, northern Sweden has been monitored since the 1980s to study long-term trends, occurrence of acid episodes, and effects of liming. The acidity in precipitation increased in the 1970s, followed by a loss of acid neutralizing capacity (ANC) and low pH in the streams. Sulfur deposition began to decrease in the 1980s, until approximately 2000, after which the decrease levelled out. Stream water sulfate concentration followed the precipitation trend but decreased more slowly and since the late 1990s a subtle increase was observed. Sulfate concentrations in the snow typically have been higher than or equal to the stream sulfate levels. However, during the period of rapid deposition decrease and also since 2005 stream sulfate has sometimes exceeded snow sulfate, indicating desorption of stored soil sulfate, possibly because of climate-related changes in run-off routes through the soil profiles, following shorter periods of frost. From 1982 to 2000, total organic carbon (TOC) increased by approximately 0.1 mg L−1 yr−1. The mean trends in sulfate and TOC from approximately 1990 until today were generally opposite. Acidic episodes with pH 4.0 at flow peaks occurred frequently in the unlimed streams, despite relatively well-buffered waters at baseflow. To evaluate the main causes for the loss of ANC during episodes, the changes in major ion concentrations during high flow episodes were evaluated. The most important factors contributing to ANC loss were dilution of base cations (Na+, K+, Ca2+, Mg2+), enrichment of organic anions and enrichment of sulfate. Wetland liming started in 1985 after which the earlier observed extreme peak values of iron, manganese and aluminium, did not reoccur. The studied area is remote from emission sources in Europe, but the critical load of acidity is still exceeded. The long-term recovery observed in the unlimed streams is thus slow, and severe acidic episodes still occur.
Highlights
Acidic deposition has caused a widespread deterioration in the water quality of lakes, water courses, and soils in large parts of Europe, primarily Scandinavia and UK, (Almer et al, 1974; Dickson, 1980; Henriksen, 1980; Wright et al, 1980), and North America, primarily in the NE USA (Hendrey et al, 1980), and Quebec and Ontario in Canada (Dillon et al, 1980; Dillon et al, 1984)
After several decades of increasing acidic deposition during the 1900s, the trend generally changed during the 1980s as a result of several international agreements to reduce sulfur emissions (e.g. UNEP, 1972; CLRTAP, 1979, 1985; Environment ’82 Committee, 1982)
In 1975, the pH of the snow was approximately 5.4 at all altitudes and was relatively unaffected by acidic deposition (Fig. 2), but the pH substantially decreased during the three following years, reaching 4.0 in 1978 at 1000 m a.s.l
Summary
Acidic deposition has caused a widespread deterioration in the water quality of lakes, water courses, and soils in large parts of Europe, primarily Scandinavia and UK, (Almer et al, 1974; Dickson, 1980; Henriksen, 1980; Wright et al, 1980), and North America, primarily in the NE USA (Hendrey et al, 1980), and Quebec and Ontario in Canada (Dillon et al, 1980; Dillon et al, 1984). After several decades of increasing acidic deposition during the 1900s, the trend generally changed during the 1980s as a result of several international agreements to reduce sulfur emissions (e.g. UNEP, 1972; CLRTAP, 1979, 1985; Environment ’82 Committee, 1982). The decrease of sulfur deposition has contributed to a recovery from acidification, which is currently being observed in the affected surface waters in several countries (Stoddard et al, 1999; Wilander and Lundin, 2000; Wilander, 2001). Sweden has been severely affected by acid rain because of the dominating igneous bedrock and relatively sparse occurrence of calcareous bedrock. Even in relatively remote areas in northern Sweden, low pH values in the snowpack were beginning to occur in the 1970s–1980s, which resulted in acidic episodes in streams
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